Optical blood flow monitoring in humans with SNSPDs and high-density SPADs

Continuous, noninvasive monitoring of cerebral blood flow (CBF) is vital for neurocritical care. Diffuse correlation spectroscopy (DCS) enables assessment of microvascular blood flow by analyzing speckle intensity fluctuations of near-infrared light. In this review, we summarize recent advances in TD-DCS using superconducting nanowire single-photon detectors (SNSPDs) at 1064 nm, as well as complementary developments in high-density CW-DCS systems using single-photon avalanche diode (SPAD) cameras. Time-gated photon detection improves depth sensitivity in TD-DCS, and the use of longer wavelengths provides advantages in tissue penetration, photon throughput, and safety margin under ANSI exposure limits. Clinically feasible SPAD-based implementations, while lacking time-of-flight resolution, enable large signal-to-noise ratio gains via massive pixel averaging and offer a room-temperature, scalable path to high-density optical tissue monitoring. Together, these developments highlight a growing set of technologies for clinical applications, including bedside brain monitoring in neurocritical care. We conclude with practical guidance on detector technologies, gating strategies, system packaging, and briefly discuss interferometric DCS and speckle contrast optical spectroscopy (SCOS) as synergistic extensions for high-resolution and high-coverage imaging.